A polyphase circuit in the form of a three-phase circuit will be described by way of example with reference to FIG. 9, which illustrates a known example of the above type of converter. The circuit of FIG. 9, which is called a single-switch step-up converter, includes a diode bridge 3, a smoothing capacitor 4, a power supply 6, reactors 51 connected to the inputs of the diode bridge 3, and a step-up chopper 52 that consists of a semiconductor switch and a diode. When the semiconductor switch is turned on, the power supply 6 is short-circuited through the reactors 51, so that the input current is formed into a desired waveform.
While the operating waveform will be described later, the switch is controlled so that non-continuous currents flow through the reactors 51 connected to the inputs of the diode bridge 3, whereby the converter develops dc current from the ac currents, while controlling the input current of each phase to be in the shape of a sinusoidal wave.
In the known circuit, the current generated upon turn-off of the semiconductor switch is not proportional to the input voltage, and therefore the input current is not formed into a sinusoidal waveform unless the output voltage is controlled to be twice or three times the maximum level of the power supply voltage. In view of the high output voltage, the switching element and diode bridge are required to have a high breakdown voltage, which causes an increase in the cost of the resulting converter. Also, the output voltage becomes as high as about 600 to 900 V, thus making it impossible for this type of converter to be used in applications where relatively low output voltage is needed. In addition, since no charge is stored in the smoothing capacitor 4 upon starting of the converter, an initial charging circuit need be provided in case of occurrence of rush current.